For those who want to understand why this is so, here it is: When a tweet begins with a @username, Twitter reads that as a reply to that @username. If it is a reply, Twitter is set up so that only those following both you and the person you are tweeting at will see it in their timeline. If there is a period (or any character) before the @username, however, Twitter simply views it as an ordinary tweet, hence seen by anyone who follows you.

(Of course, the question of whether your followers want to see whatever is happening beyond that period is a question for you to decide for yourself. The purpose of this chart is simply to help you know what you're doing.)